For the latest on the bushfire threat

RFS spokesman Joel Kershaw says the alertness of crews and the general public today has helped keep the situation contained so far.

"A number of the blazes we've seen today in Sydney and across the Hunter region have been brought under control pretty quickly today because of the prompt and swift response of firefighters and members of the public phoning these fires through to triple-0 as soon as they see them," he said.

"We're still looking very carefully at the weather. We've still got the next four to six hours where those warm conditions, hot dry winds and the humidity will be quite low until that southerly change comes through."In terms of Sydney, we're not expecting to see cooler conditions until after midnight this evening. Once those temperatures cool down however, there’s a strong southerly change that comes through with that.

"We’ll be watching that very carefully because if we do have fires burning that then becomes problematic for us in terms of erratic fire behaviour and the direction of those fires shifting."

The weather bureau had been forecasting the mercury would reach 39 degrees Celsius in Sydney which would broken the previous October record of 38.2C set in 2008.

But the temperature fell short of that to peak at 37.8C at Sydney Airport.

Earlier today RFS Commissioner Shane Fitzsimmons said fire conditions could deteriorate further, with the potential to become catastrophic.

"We do have a landscape that is highly susceptible to ignitions. Fires will start easily and spread very quickly under the sorts of conditions we're talking about today," Commissioner Fitzsimmons warned.

"Fires that start to take hold in those sort of areas are likely to be uncontrollable and our focus will be on preserving life and trying to protect properties and infrastructure as fires run.

"Those hot dry winds coming across the continent will see wind speeds ahead of the change coming through at about 130 kilometres an hour across the ranges.

"We could see gusts up to 70 to 80 km throughout Sydney and behind that change we're expecting a southerly to kick in with wind strengths up to 100 kilometres an hour.

"We can expect to see five, six, seven hours of extreme fire danger ratings and those conditions will strengthen as that frontal pattern starts to move from the south of the state right up across those extreme weather areas."